Electric shutter operating circuit for cameras

ABSTRACT

A ratio detector detects the ratio between the resistances of internal and external photoconductive elements. The output of the ratio detector is converted to a corresponding resistance value, and current determined by the ratio is applied during a definite interval to a first charge capacitor, acting as a memory for the ratio. A second capacitor is charged through the external photoconductive element concurrently with the opening of the shutter of the camera. The outputs from the first and second capacitors are compared to produce an output pulse when these two outputs coincide and to close the shutter.

Fujii et al.

[ 1 3,731,604 [451 May 8,1973

[ 1 ELECTRIC SHUTTER OPERATING CIRCUIT FOR CAMERAS Inventors: Masaya Fujii, Tokyo; Osamu Ichihashi, Hamamatsu, both of Japan [73] Yashica Company, Ltd., Tokyo,

Japan Filed: June 10, 1971 Appl. No.: 151,719

Assignee:

[30] Foreign Application Priority Data June 12, 1970 Japan ..45/50521 [56] References Cited UNITED STATES PATENTS Nobusawa ..95/53 EB Sato ..95/10 CT THROUGH THE l i l & LENS LIGHT 1/1972 11/1971 Wagner et a1. ..95/10 CT Primary ExaminerSamuel S. Matthews Assistant Examiner-Russell B. Adams, Jr. Att0rneyFlynn & Frishauf 57] ABSTRACT A ratio detector detects the ratio between the resistances of internal and external photoconductive elements. The output of the ratio detector is converted to a corresponding resistance value, and current determined by the ratio is applied during a definite interval to a first charge capacitor, acting as a memory for the ratio. A second capacitor is charged through the external photoconductive element concurrently with the opening of the shutter of the camera. The outputs from the first and second capacitors are compared to produce an output pulse when these two outputs coincide and to close the shutter.

I 5 Claims, 3 Drawing Figures INTERNAL A PHOTUBUNDUGTIVE D mum I LA JZ RESIS c LIGHT l l l t i l l i n T E CONVERTER EXTERNAL B PHUTDBUNDUBIIVE l ELEMENT H commas F MEMORY SHUTTER CIRCUIT I 30mm Yashuhiro ..95/l0 CT PAIENTEDHAY 81973 3.731.604

SHEET 1 OF 2 Mx-vnnoucu THE NH LENS ucm TIMED mm A pnmncnuuucnw F/ l n ELEMENT /--EXTERNAL 1 F??? LIGHT 'E RQFE E GUNVEHIER EXTERNA arnumcuunu f ELEMENT H comma F mum A PUTENIIMUF 00mm n n s B PUTENTIALUF comma 13 14 [1 sum I5 w I] swam 1!! vnmsiurm 4 T V2 vumsmrm h G umPuIm n muslsnm n F H UUTPUIUFFET h F I OUTPUT TBANSIS ELECTRIC SHUTTER OPERATING CIRCUIT FOR CAMERAS BACKGROUND OF THE INVENTION This invention relates to an electric shutter operating circuit for a single lens reflex type camera for automatically determining the shutter speed in accordance with the brightness of an object to be photographed, and more particularly to an electronic shutter which determines the exposure time in accordance with the light transmitted through the objective lens.

A photoconductive element is disposed on the optical axis of the lens to measure the brightness or lumens near the light receiving surface, so that this type of shutter can accurately measure the lumens at the film plane even when the lens is exchanged or a filter is used.

In cameras of the SLR type it is necessary to retract the photoconductive element from the optical axis when the shutter is opened. For this reason, in order to obtain an appropriate shutter timing it is necessary to memorize the measured light value at a time before the shutter is opened.

U.S. Pat. No. 3,324,779 shows a system in which light is measured by a photoconductive element which is disposed on the optical axis a signal representative of light is stored before opening of the shutter in a capacitor, and the shutter time is determined in accordance with the stored valve. While this system is advantageous in that it can store the measured value of the light with a simple element the stored value is the value that has been measured before opening of the shutter and does not represent the value prevailing during an interval in which the shutter is actually opened. For this reason, where the brightness of the object varies the stored value of the light is different from the actual brightness of the object thus it is not possible to obtain correct exposure.

It is an object of this invention to provide an electronic shutter operating circuit which does not use any moving parts and hence is compact, inexpensive and shock proof and which can assure proper exposure even when the brightness of the object varies.

SUBJECT MATTER OF THE INVENTION An external photo-conductive element disposed in front of the camera and is responsive to ambient light; an internal photo-conductive element is responsive to light through the lens. A ratio detector detects the ratio between the resistances of the internal and external photoconductive elements; a converter typically a transistor, converts the output of the ratio detector into a corresponding current value which is applied, under control of a timing switching clement for a definite intcrval, to a first charge accumulator typically a capacitor to he charged. A second charge accumulator (capacitor) is charged through the external photoconductive element concurrently with the opening of the shutter ofthe camera, and when the internal photoconductive element is out of the path of light. A comparator compares the outputs from the first and second charge accumulators to produce an output when these two outputs coincide. A shutter control is responsive to the output from the comparator to close the shutter.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram to explain the principle of the electric shutter operating circuit embodying this invention;

FIG. 2 shows a connection diagram of one example of the novel shutter operating circuit; and

FIG. 3 shows waveforms at various portions of the circuit shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT The electric shutter operating circuit shown in FIG. 1 comprises an internal photoconductive element A disposed in the body of a camera on the optical axis thereof, an external photoconductive element B disposed in front of the camera and responsive to incident light, a ratio detector C for obtaining an electrical quantity corresponding to the ratio between the resistances of the internal and external photoconductive elements A and B, a timed switching elements D which turns ON for a definite interval of time, a converter E which receive, the output from the switching element D for converting the output of the detector C into an electrical signal having a value representative of this ratio, e.g., a certain current, a first memory or charge accumulator F which stores the output from the switching element D over a predetermined interval, a second memory or a charge accumulator G which is charged through the external photoconductive element B at the time of commencing photographing and which stores a value representative of then incident light, a comparator H which provides an output when the output from the second memory G coincides with that of the first memory F and a shutter control I responsive to the output of the comparator H to close the shutter which has been opened previously.

The shutter operating circuit of the construction describe above operates to compare the lumens on the outside with those near the light receiving surface and to determine the shutter operating time. The comparison is performed by the internal and external photoconductive elements A and B and by the ratio detector C. Thus voltages Vin and Vout corresponding to the resistances of the internal and external photoconductive elements A and B respectively, are apllied to the ratio detector C to obtain a ratio P=Vin/Vout. For this reason, although at the time of photographing the internal photoconductive element A is rendered inoperative it is possible to obtain a valve Vin P. Vout corresponding to the resistance variation of the internal photoconductive element irrespective of the change in the brightness of the object.

With reference now to the operation for determining the shutter time, the output of the switching-element D is charged to the first memory F for a definite time through converter E. The quantity being charged corresponds to the ratio P. The charge in the first memory F is preserved or stored concurrently with the opening of the shutter a charge is transferred to the second memory F through the external photoconductive element B. Then comparator H compares the output from the first memory F with that of the second memory G to provide an output to the shutter control I when these two output coincide with each other. Under this condition Vin P. Vout, and, assuming that the ratio of light did not change, Vout can be determined. In response to the output from the comparator H the shutter control I closes the shutter by unlocking it. In this manner, this shutter operating circuit operates to open the shutter for an interval corresponding to Vin I. Vout.

an internal photoconductive element RA is disposed in the body of the camera on the optical axis of the lens thereof, an external photoconductive element RB is disposed in front of the camera, and measures ambient light A first transistor T, for produces a current corresponding to the ratio of the resistance of the internal photoconductive element RA to that of the external photoconductive element RB (switches 8,, S at terminals b). A capacitor C, is connected through the external photoconductive element RB when switch S transfers to terminal C to determine the time the shutter is to remain OPEN. A capacitor C is connected to be charged for a definite time interval through the first transistor T, by the current therethrough. There are also A second transistor T compares the voltages on capacitors C, and C Transistors T and T., control charging current flow to capacitor C transistors T,, and T,, are connected as a monostable multivibrator to determine the charging time (but not the rate) of the capacitor C A transistor T,, energized by the output from transistor T controls shutter closing. An electromagnet M is provided to unlock the shutter for closing the same, the electromagnet M being controlled by a transistor T The output from transistor T is supplied to the gate electrode of a field effect transistor FET to supply a trigger voltage to the base electrode of transistor T,,. C C and C represent coupling capaci tors, R, is a resistor for applying a bias voltage to diodes D, and D R a high resistance resistor for supplying a substantially constant current to the internal photoconductive element RA, R and R are bias resistors for transistor T and R and R fixed coupling resistors. The shutter operating circuit is energized by a source B, 5,, S S 8., and 8,, are switches interlocked with the camera shutter release button, not shown. When the release button is depressed slightly the movable switch contacts a are thrown to stationary contacts b from the neutral positions, and that when the release button is depressed further, so that the shutter is opened, the movable contacts a are thrown to the stationary contacts c. When the shutter is closed the movable contacts a are restored to the neutral position shown in the drawing.

When the release button is depressed slightly, the movable contacts a of switches S, to S, are thrown to contacts b at a time t, as shown by FIG. 3A. As a result, a circuit extending through internal photoconductive element RA, external photoconductive element RB and base electrode of transistor T, is established whereby the following relationship is established between the collector current I,- and the base current 1,, oftransistor T,.

Ic=hFE IB accordingly the resistance RcE between the collector and emitter electrodes is given by RcE= VcE/Ic= VcE/hFE'IB 2) where VcE represents the collector-base resistance. Since the resistance of resistor R is high the collector current 10 is given by the following equation.

Ic=hFE=hFEI'RA/RB 3 when I represents the current flowing through the internal photoconductive element RA. Substitution of equation 3 into equation 2 gives RcE= VCE RB/hFE'I'RA =A'(RB/RA) 4 where A represents a constant.

Thus, when the movable contacts a of switches S, and S are thrown to contacts b the emitter-collector resistance RcE assumes a value corresponding to the ratio between resistances RB of the internal and external photoconductive elements RA and RB.

As is well known in the art when the movable contact is thrown to contact b transistor T of the monostable multivibrator comprised by transistors T,, and T,,, resistors R through R, and capacitors C and C is turned ON (at a time t, as shown in FIG. 3, line C), whereas transistor T,, is turned OFF as shown by t, of FIG. 3, line D. As a result of the voltage drop across resistor R transistor T, is turned ON. At the same time as the voltage across resistor R,,, reduces to substantially zero, transistor T is turned OFF. The interval during which transistor T is ON, transistor T,, is OFF, transistor T,, is ON, and transistor T is ON is a constant interval W determined by the values of resistor R and capacitor C, as shown by time instants t, and t of FIG. 3D.

When transistors T,, and T,, are ON for the definite interval W, a circuit is established which can be traced from one pole of source B to the other through transistor T,, contact 11 of switch S capacitor C contact b of switch S, and transistor T to charge capacitor C, for the definite interval W by a current corresponding to the ratio of resistances of the internal and external photoconductive elements RA and RB as shown by t, and of FIG. 3B.

As the shutter release button is depressed further, movable contacts a of all switches S, through S and thrown to contacts c while at the same time the shutter, not shown, is opened. Then a closed circuit is formed across source B through the external photoconductive element RB, a contact 0 of switch S contact 0 of switch S and capacitor C, to charge it by a current corresponding to the light received by the external photoconductive element RB. Further, closure of contact c of switches S and S, connects capacitor C between the emitter electrode of transistor T and the ground. Thus, the voltage across capacitor C, builds up as shown by FIG. 3F and when the voltage V, across capacitor C, coincides with the voltage V, across capacitor C transistor T applies its output to the base electrode of transistor T, through resistor R, as shown in FIG. 36. Then transistor T, is turned ON to apply its output to the gate electrode of the field effect transistor FET thus causing it conductive for a short internal, as shown by FIG. 3H, Conduction of the field effect transistor FET renders conductive transistor T for a short interval thus energizing the electromagnet M, as shown by FIG. 3I, thus unlocking the shutter to close the same. Upon closure of the shutter, switches S,

through S are returned to their neutral positions thus completing the photographing operation. in this manner, the shutter is maintained open for an interval between 2 and t as shown in FIG. 36.

Thus, with the novel shutter operating circuit, although the internal photoconductive element does not receive light as the time of photographing, it is possible to obtain a value equal to that measured by the internal photoconductive element while the shutter was closed by multiplying the value measured by the external photoconductive element with the ratio between resistance values of the internal and external photoconductive elements RA and RB, In this embodiment, when a photographic film of different sensitivity is used, the shutter timing can be obtained by adjusting the quantity of light incident upon the external photoconductive element RB or by varying capacitor C or resistor R of the monostable multivibrator.

Although in the embodiment described above, transistors are used as switching means and a monostable multivibrator is used to turn ON the transistor for a definite interval, the invention is not limited to these particular means but many other devices may be used. Instread of determining the instant at which the actual voltages of capacitors C and C coincide, it is also possible to determine an instant at which the voltage across capacitor C coincides with a voltage proportional to the voltage across capacitor C According to this invention the ratio of the light quantities measured by the internal and external photoconductive elements is determined to determine the shutter time by said ratio and the light quantity measured by the external photoconductive element so that it is possible to accurately determine the brightness of the object. Thus, even when the brightness of the object varies it is possible to obtain correct exposure by rapidly following such variation. Further this invention enables the measurement of the lumens equivalent to the internal measurement at the time of photographing by using an electronic circuit alone, that is within the necessity of utilizing any moving part such as an iris diaphragm and an operating to provide an appropriate shutter speed corresponding to the measured value. For this reason, it is possible to construct the shutter operating circuit as a compact and inexpensive unit. Moreover such circuit can be used instantly and is shock proof.

What is claimed is:

1. An electric shutter operating circuit for a camera comprising an external photoconductive element (RB) disposed in front of said camera and responsive to incident light;

an internal photoconductive element (RA) disposed in said camera and responsive to light passing through the camera lens;

a ratio detector and converter (C, E; T,, R to detect the ratio between resistances of said external and internal photoconductive elements and convert the output of said ratio detector into a corresponding electrical signal;

a timed switching element (D; T T providing a timing control current for a definite interval;

a first charge accumulator (F; C connected to said signal to memorize said signal during said definite interval under control of said switching element;

a second charge accumulator (C to accumulate the charge representative of current through the external photoconductive element (RB) concurrently with the opening of the shutter of said camera, and

5 interruption of application of light to said internal photoconductive element (RA);

a comparator (H; T to compare the outputs from said first and second charge accumulators (C C to produce an output when said two outputs coin- 10 cide;

and a shutter control (1; T FET, T M) responsive to the output from said comparator (H, T to close said shutter.

2. Circuit according to claim 1, wherein the timed switching element comprises a monostable multivibrator circuit.

3. Circuit according to claim 1, wherein the first charge accumulator comprises a capacitor (C and the ratio detector comprises a transistor (T having its base connected to a circuit formed of the internal and external photoconductive elements (RA, RB) in series, anda resistor (R connected from the junction of the series connected photoconductive elements to one of the other electrodes of the transistor (T so that current flow through the emitter-collector path of the transistor (T will be representative of the ratio of resistances of the internal and external photoconductive elements, said capacitor (C being connected in circuit with the emitter-collector path to be charged thereby, the charge representing a stored memorized electrical value representative of the ratio of resistances of said photoconductive elements.

4. Circuit according to claim 1, wherein the first and second charge accumulators comprise capacitors (C 1);

and the comparator comprises a transistor (T connected to have one of the capacitors (C connected across its base and one other terminal (emitter) and the other capacitor (C connected in series with said other terminal (emitter);

and output means connected to the third terminal (collector) of the transistor (T and providing said output upon coincidence of charge on said capacitors.

5. Circuit according to claim 3, wherein said second charge accumulator comprises a second capacitor 5 0 (C1 and the comparator comprises a second transistor (T connected to have the second capacitor (C connected across its base and one other terminal (emitter), and the first capacitor (C connected in series with said other terminal (emitter); output means connected to the third terminal (collector) of the second transistor (T and providing said output upon coincidence of charge on said capacitors (C C and selective switching means (S S 8;, T T coupled to operation of the shutter, selectively con necting said first capacitor (C in the emitter-collector circuit of the first transistor (T to store the charge representative of said resistance ratio, and, after said definite interval, being connected in the emitter-collector circuit of said second transistor (T to provide a stored reference value of said resistance ratio, for comparison with the resistance of the external photo-conductive clement (RB) only; as represented by the charge being applied to said second capacitor (C,) 

1. An electric shutter operating circuit for a camera comprising an external photoconductive element (RB) disposed in front of said camera and responsive to incident light; an internal photoconductive element (RA) disposed in said camera and responsive to light passing through the camera lens; a ratio detector and converter (C, E; T1, R2) to detect the ratio between resistances of said external and internal photoconductive elements and convert the output of said ratio detector into a corresponding electrical signal; a timed switching element (D; T5, T6) providing a timing control current for a definite interval; a first charge accumulator (F; C2) connected to said signal to memorize said signal during said definite interval under control of said switching element; a second charge accumulator (C1) to accumulate the charge representative of current through the external photoconductive element (RB) concurrently with the opening of the shutter of said camera, and interruption of application of light to said internal photoconductive element (RA); a comparator (H; T2) to compare the outputs from said first and second charge accumulators (C2, C1) to produce an output when said two outputs coincide; and a shutter control (I; T7, FET, T8, M) responsive to the output from said comparator (H, T2) to close said shutter.
 2. Circuit according to claim 1, wherein the timed switching element comprises a monostable multivibrator circuit.
 3. Circuit according to claim 1, wherein the first charge accumulator comprises a capacitor (C2) and the ratio detector comprises a transistor (T1) having its base connected to a circuit formed of the internal and external photoconductive elements (RA, RB) in series, and a resistor (R2) connected from the junction of the series connected photoconductive elements to one of the other electrodes of the transistor (T1), so that current flow through the emitter-collector path of the transistor (T1) will be representative of thE ratio of resistances of the internal and external photoconductive elements, said capacitor (C2) being connected in circuit with the emitter-collector path to be charged thereby, the charge representing a stored memorized electrical value representative of the ratio of resistances of said photoconductive elements.
 4. Circuit according to claim 1, wherein the first and second charge accumulators comprise capacitors (C2, C1); and the comparator comprises a transistor (T2) connected to have one of the capacitors (C1) connected across its base and one other terminal (emitter) and the other capacitor (C2) connected in series with said other terminal (emitter); and output means connected to the third terminal (collector) of the transistor (T2) and providing said output upon coincidence of charge on said capacitors.
 5. Circuit according to claim 3, wherein said second charge accumulator comprises a second capacitor (C1); and the comparator comprises a second transistor (T2) connected to have the second capacitor (C1) connected across its base and one other terminal (emitter), and the first capacitor (C2) connected in series with said other terminal (emitter); output means connected to the third terminal (collector) of the second transistor (T2) and providing said output upon coincidence of charge on said capacitors (C2, C1); and selective switching means (S1,S2, S3, T3, T4), coupled to operation of the shutter, selectively connecting said first capacitor (C2) in the emitter-collector circuit of the first transistor (T1) to store the charge representative of said resistance ratio, and, after said definite interval, being connected in the emitter-collector circuit of said second transistor (T2) to provide a stored reference value of said resistance ratio, for comparison with the resistance of the external photo-conductive element (RB) only, as represented by the charge being applied to said second capacitor (C1) 